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Chances are that if you don’t buy into any medical conspiracy theories, someone else close to you does. A new survey from the University of Chicago has found that nearly half of all Americans believe in at least one conspiracy theory, ranging from cancer-causing cell phones to the government’s indifference to “harmful” vaccines.

Conspiracy theories aren’t just fanciful ideas; they approach delusion, argues study leader Dr. J. Eric Oliver, who researches political psychology and public opinion at Chicago. According to Oliver, when people believe in conspiracy theories they demonstrate a fundamental unwillingness to accept scientific reason. The ideas are challenging to them, so they bristle — eventually forming conclusions just for the sake of knowing something.

"Science in general — medicine in particular — is complicated and cognitively challenging because you have to carry around a lot of uncertainty," Oliver told Reuters. "To talk about epidemiology and probability theories is difficult to understand as opposed to 'if you put this substance in your body, it's going to be bad.’”

For example, it’s easy to argue, as talk show host Jenny McCarthy famously has, that vaccine rates rising alongside autism diagnosis means vaccines somehow cause autism. That correlation is easy. It’s much harder, and far less satisfying, to wrestle with the large problem of over-diagnosis, rising ADHD rates, shortening attention spans, less-active youth culture, and cost-benefit analyses of not getting vaccinated, and not instantly be able to take a side.

In the study, Oliver’s team looked at data from 1,351 adults who answered an online survey between August and September 2013. Participants read six conspiracy theories. They reported whether they had heard the theory and if they agreed or disagreed with it.

The six theories included: the government’s conspiracy to infect black people with HIV, to deprive citizens of access to alternative medicines, its ambivalence toward cell phones’ cancer-causing properties, its use of genetically modified organisms to shrink the population, that vaccines cause autism, and that fluoridation enables companies to pollute water supplies.

Overall, 49 percent of respondents agreed with at least one of the theories. Some 37 percent of people said they believed the government tries to limit access to natural cures, while less than a third were willing to say they actively disagreed with the theory. When it came to autism-causing vaccines, 20 percent agreed and 44 percent disagreed. Only 69 percent of people had ever heard of the theory.

Oliver and his colleagues are hesitant to write off the conspiracy theorists completely. "Although it is common to disparage adherents of conspiracy theories as a delusional fringe of paranoid cranks,” they wrote, “our data suggest that medical conspiracy theories are widely known, broadly endorsed, and highly predictive of many common health behaviors.”

But does endorsement imply truth? One of the beauties of science is that it’s true even if you don’t believe it. Dogma has never had a place in the discipline, and a growing number of people espousing a certain falsehood only serves to entrench us in our collective ignorance. Rightness is found in fact, not in enough people being wrong.

"It's important to increase information about health and science to the public," Oliver told Reuters. "I think scientific thinking is not a very intuitive way to see the world. For people who don't have a lot of education, it's relatively easy to reject the scientific way of thinking about things."